Field of the Invention
The invention is concerned with the combined preparation of at least butene and octene from ethene.
Discussion of the Background
Hydrocarbons are chemical compounds which consist exclusively of carbon C and hydrogen H. Alkenes (synonym: olefins) are hydrocarbons which have a C═C double bond in the molecule. Alkanes (synonym: paraffins), on the other hand, are hydrocarbons which have only single bonds. They are therefore also referred to as saturated.
In organic chemistry, hydrocarbons are frequently designated according to the number of carbon atoms which they have per molecule, by the respective class of substances being preceded by the prefix Cn. Here, n is the respective number of carbon atoms in a molecule. Thus, C4-olefins are substances from the class of alkenes having four carbon atoms. C8-olefins correspondingly have eight carbon atoms per molecule. Where the prefix Cn+ is used in the following, it refers to a class of substances which have more than n carbon atoms per molecule. A C4+-olefin accordingly has at least five carbon atoms.
The simplest olefin is ethene (ethylene). It has two carbon atoms. Ethene is an important basic chemical and is therefore prepared in large quantities. This is usually effected by steam cracking of naphtha. In addition, it can be obtained by dehydrogenation of ethane, which in turn is a constituent of natural gas. Owing to the increasing exploitation of unconventional sources of natural gas and decreasing recovery of petroleum, the proportion of ethene based on natural gas is steadily increasing.
C4-olefins encompass the four isomeric materials 1-butene, cis-2-butene, trans-2-butene and isobutene. 1-Butene and the two 2-butenes belong to the group of the linear butenes, while isobutene is a branched olefin. The linear C4-olefins 1-butene, cis-2-butene and trans-2-butene are often summarized as “n-butene” in the literature. Depending on the thermodynamic circumstances, the four isomeric C4-olefins usually occur together. For this reason, no distinction between singular and plural is made when the term “butene” is used. When reference is made here to “butene” with no further details being specified, what is meant is a linear alkene having four carbon atoms (or n-butene) or a mixture containing different isomeric alkenes having four carbon atoms.
A current overview of the chemical and physical properties of butenes and also the industrial processing and utilization thereof is given by:    F. Geilen, G. Stochniol, S. Peitz and E. Schulte-Koerne: Butenes. Ullmann's Encyclopedia of Industrial Chemistry. (2013)
Butenes are nowadays predominantly obtained in the cracking of petroleum fractions in a steam cracker or in a fluid catalytic cracker (FCC) and are used as intermediate for the preparation of a variety of industrial chemicals.
In the following, a “hexene” is an olefin having six carbon atoms or a mixture containing a plurality of different C6-olefins. For this reason, no distinction is made between singular and plural when using the term “hexene”. The C6-olefins include the eighteen isomers 1-hexene, (E)-2-hexene, (Z)-2-hexene, (E)-3-hexene, (Z)-3-hexene, 2-methyl-1-pentene, 2-methyl-2-pentene, (R)-3-methyl-1-pentene, (S)-3-methyl-1-pentene, (E)-3-methyl-2-pentene, (Z)-3-methyl-2-pentene, 4-methyl-1-pentene, (E)-4-methyl-2-pentene, (Z)-4-methyl-2-pentene, (3S)-2,3-dimethyl-1-butene, (3R)-2,3-dimethyl-1-butene, 2,3-dimethyl-2-butene and 3,3-dimethyl-1-butene.
However, only the substances 1-hexene and 4-methyl-1-pentene, which are used as monomers or comonomers in the production of plastics, are of industrial interest. For this purpose, they are prepared from ethene or from the C3-olefin propene by oligomerization. The oligomerization will be explained in detail below.
For the purposes of the present invention, octene is an olefin having eight carbon atoms or a mixture containing a plurality of different C8-olefins. The C8-olefins include a large number of isomers which are too many to list here. An industrially important representative of the C8-olefins is 1-octene which is prepared by oligomerization of ethene and is used as comonomer in polyethylene.
An alternative way of preparing octene is dimerization of n-butene. The mixture of olefins having eight carbon atoms which is formed here is referred to as dibutene, and is thus a particular octene within the meaning of the terminology employed here. Dibutene is distinguished by the isomer distribution, in terms of which it differs from other octene mixtures.
Depending on the way in which the individual n-butene molecules are joined in the course of the oligomerization, an oligomer having a different degree of branching is obtained. The degree of branching is described by the iso index, which states the mean number of methyl groups per C8 molecule in the isomer mixture. The iso index for dibutene is defined as follows:Iso index=(proportion by weight of methylheptenes+2*proportion by weight of dimethylhexenes)/100Thus, n-octenes contribute 0, methylheptenes contribute 1 and dimethylhexenes contribute 2 to the iso index of a product mixture of C8-olefins. The lower the iso index, the less branched are the molecules in the mixture. The opposite of branched in this context is linear. A product with high linearity accordingly has a low degree of branching.
A low degree of branching is always important when the olefin mixture is to be used as starting material for preparing plasticizers. Scientific studies demonstrate that the degree of branching of olefin mixtures which are processed further by hydroformylation, hydrogenation and esterification to give plasticizers is critical to the properties and quality of the plasticizer.
The iso index which a C8-olefin mixture has to achieve in order to be able to serve as starting material for high-quality plasticizers depends on the respective requirements of the plasticizer customers and changes over time. At present, an iso index of less than 1.1 is usually required.
For the purposes of the present invention the oligomerization which has now been mentioned a number of times is the reaction of hydrocarbons with themselves, forming corresponding longer-chain hydrocarbons. Olefins having from two to eight carbon atoms can be oligomerized very readily.
Thus, for example, an olefin having six carbon atoms (hexene) can be formed by oligomerization of two olefins having three carbon atoms. The oligomerization of two molecules with one another is also referred to as dimerization. If, in contrast, three olefins having three carbon atoms are joined to one another (trimerization), the result is an olefin having nine carbon atoms. If n-butene is subjected to an oligomerization, essentially olefins having eight carbon atoms (more precisely: dibutene) and also olefins having twelve carbon atoms (C12-olefins, “tributene”) and to a lesser extent olefins having more than twelve carbon atoms (C12+-olefins, called “tetrabutene”) are formed.
One process employed in industry for preparing dibutene by oligomerization of n-butene is the OCTOL® process. Detailed description thereof can be found in the nonpatent literature, for example in:    B. Scholz: The HÜLS OCTOL Process: Heterogeneously catalyzed dimerization of n-butenes and other olefins. DGMK conference in Karlsruhe, published in Erdöl, Erdgas, Kohle, April 1989, pages 21 and 22.    R. H. Friedlander, D. J. Ward, F. Obenaus, F. Nierlich, J. Neumeister: Make plasticizer olefins via n-butene dimerization. Hydrocarbon Processing, February 1986, pages 31 to 33.    F. Nierlich: Oligomerize for better gasoline. Hydrocarbon Processing, February 1992, pages 45 to 46.
In the patent literature, an oligomerization based on the OCTOL® process is described, for example, in DE102008007081A1. EP1029839A1 is concerned with the fractionation of the C8-olefins formed in the OCTOL® process.
The completely heterogeneously catalyzed OCTOL® process gives a dibutene which has a low degree of branching and is highly suitable for the preparation of plasticizers. Heterogeneously catalyzed means that the catalyst is present as a solid in the liquid or gaseous reaction mixture. The fluid reactants thus flow around the catalyst and the catalyst remains in the reactor. Since the OCTOL® process has been optimized on the processing of C4-olefins as feedstock, it is dependent on the availability of butenes as feedstock. Other raw material sources cannot be processed without extra expense. Since C6-olefins are not obtainable by oligomerization of C4-olefins, the preparation of hexene with the OCTOL® process is not possible. Dibutene, tributene and tetrabutene are exclusively formed. The OCTOL® process is inflexible in this respect.
A greater flexibility with respect to the target product can be achieved by cooligomerization. The term cooligomerization refers to the simultaneous oligomerization of a plurality of substrates in one reaction vessel. Thus, EP2582648B1 describes the cooligomerization of butene and octene to give dodecene (C12-olefin). As in the case of any oligomerization, which olefin reacts with which is not precisely known in a cooligomerization: In the example of EP2582648B1, a dodecene can be formed both from three butenes and also from a butene and an octene. From a chemical point of view, any oligomerization can be considered to be a cooligomerization. From an industrial point of view, on the other hand, a cooligomerization is present only when at least two olefins having different numbers of carbon atoms are introduced into a common reactor. In the choice of terminology, it is thus the controllable introduction of the starter materials which matters, not the reaction which actually takes place. The process described in EP2582648B1 is even more flexible with respect to its C8 and C12 target products than an oligomerization which uses exclusively C4 as starter material. Nevertheless, the preparation of hexene is likewise not possible. This is only possible by oligomerization of ethene and/or propene.
WO2005/123884 discloses the combined preparation of 1-octene and 1-hexene by tetramerization and trimerization of ethylene. For this purpose, two different homogeneous catalysts, namely a first catalyst for tetramerization and a second catalyst for trimerization, are provided in a common reaction vessel. Since the homogeneous catalysts used are dissolved in the reaction mixture, they must either be removed therefrom or remain therein. The latter case is then no problem if the reaction mixture is used as comonomer in a polyethylene synthesis. In the preparation of polyethylene, exclusively homogeneous catalysts are in fact used which remain in the polymer and are therefore lost. If however the 1-hexene and 1-octene prepared were required to be specially separated and as far as possible obtained as pure substances, the dissolved homogeneous catalysts had to be first laboriously removed. The process is therefore hardly suitable for the isolated preparation of 1-octene and 1-hexene and makes sense industrially only in connection with polyethylene synthesis.
Furthermore, this process also does not appear to be suitable for preparing C8-olefins for use as starting material for plasticizers: Although up to 52% by weight of C8-olefins are obtained in combined tetramerization and trimerization, the degree of branching is not specified precisely. Moreover, the process is optimized for the production of the comonomer 1-octene, viz. a C8-olefin which in any case is not very suitable for plasticizer production. It is therefore not possible to see that the C8-alkenes achieve an iso index which qualifies them as starting material for plasticizer production. In addition, the homogeneously dissolved catalyst would definitely have to be separated off in this use since the subsequent hydroformylation is likewise homogeneously catalyzed and is sensitive to interference caused by extraneous catalysts introduced by entrainment.
What has just been said also applies to the process disclosed in WO2005/123633 for the oligomerization of ethylene, which is carried out in the presence of cyclohexane. The cyclohexane serves as solvent and is intended to reduce the deactivation of the homogeneous catalyst used or its activator.
A similar situation also applies to US2013/0066128 A1 which is concerned with the homogeneous oligomerization of ethene in n-heptane.
The problem of separating off the catalyst does not arise in heterogeneously catalyzed processes in which the catalyst is present as a solid and remains in the reactor. Ethylene oligomerization over a solid Si/Al/Ni system is described in U.S. Pat. No. 8,637,722B2. However, this process takes place in the gas phase, which is disadvantageous in terms of the utilization of space by the reactors. In addition, the established process steps of further processing of butenes and octenes take place in the liquid phase, so that this gas-phase process is not readily compatible with existing technology. A need to liquefy the butenes and octenes obtained in the gas phase requires additional energy.
The gas-phase process shown in WO2010/117539A1 for the oligomerization of ethylene diluted in a FCC gas over a zeolitic Ni catalyst can also not be incorporated directly into an established production run for C4/C8 utilization.
The same is also true for the heterogeneous gas-phase oligomerization of ethene over a nickel-containing zeolite described in U.S. Pat. No. 4,717,782. The feed mixture can also comprise C4-paraffins and inert gases. U.S. Pat. No. 8,637,722 also describes the oligomerization of ethene in the gas phase over a heterogeneous catalyst of Ni/Al on a support made of Al2O3/SiO2. Inert gases such as nitrogen, argon or helium may be present.
A mixed form between heterogeneous and homogeneous C2-oligomerization is shown in US2013/0158321A1. Here, ethene is firstly dimerized homogeneously to butenes and these are then converted to octenes by heterogeneous catalysis over a solid nickel catalyst. Both reaction stages take place in the liquid phase in the presence of hexane. The reaction discharge from the first stage has to be neutralized with base and freed from the homogeneous catalyst (triethylaluminium) by distillation. In industrial practice, this is very complex.
U.S. Pat. No. 2,581,228 describes the heterogeneously catalyzed oligomerization of ethene in the presence of an inert solvent. The solvent should be a relatively high-boiling, inert material, preferably a relatively high-boiling alkene or cycloalkene. The catalyst used is a nickel/aluminium system on silica gel. The reaction mixture is a slurry from which the gel-like catalyst can be recovered. Corresponding expenditure on apparatus will be entailed for this.